Solvent properties of ionic liquids on complex formation of Ni(II) ion with molecular liquids

Room-temperature ionic liquids (ILs) attract much attention as green solvents because of their negligible volatility and non-flammability. In analytical chemistry, the application of ILs to chemical separation and solvent extraction has been investigated. However, there have been a smaller number of reports on the mechanisms of chemical equilibria in the solvents of ILs at a molecular level. In this review, the series of our investigations on the complex formation of Ni²⁺ ion with molecular liquids (MLs) in imidazolium-based ILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C₂mim][TFSA]) and 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)amide ([C₈mim][TFSA]) was summarized. The MLs involved methanol (MeOH), ethanol (EtOH), acetonitrile (AN), and dimethyl sulfoxide (DMSO). In [C₂mim][TFSA], the Ni²⁺-ML complexes are stabler in the sequence of DMSO >> AN > EtOH ≈ MeOH complexes, which almost depends on the electron donicities of MLs. In contrast, in [C₈mim][TFSA], the sequence changes to AN > DMSO > EtOH > MeOH complexes, despite of the lowest electron donicity of AN. Thus, only the electron donicities of MLs do not decide the stability of the complexes. The reasons for the stabilities of the Ni²⁺-ML complexes were interpreted in terms of the microscopic interactions between ML and the ILs' cation observed by the infrared (IR) and ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopic techniques. Furthermore, to clarify the effects of ML cluster formation on the complex formation mechanisms, the heterogeneous mixing between IL and ML were quantitatively evaluated by small-angle neutron scattering (SANS) at the mesoscopic scale.